Off-pump coronary artery bypass grafting (OPCABG) is fast becoming widely adopted technique for surgical revascularization of the heart. However, OPCABG presents with unique technical and hemodynamic challenges which require additions to conventional monitoring techniques. Transesophageal echocardiography (TEE) provides reliable and real-time information to monitor these challenges during OPCABG. TEE is an invaluable diagnostic tool for real-time imaging during OPCABG. It is beneficial not only in the intraoperative period but also in the postoperative care units for better patient outcomes.

Routine TEE during cardiac surgery has shown to reduce patient morbidity and mortality and improve patient outcome.[1] It is beneficial in high-risk patients undergoing CABG. Savage et al. have shown that TEE changed surgical management in 57% and anesthetic management in 73% of CABG patients.[2] Skinner et al. and Klein et al. have also shown that preoperative studies alone may not accurately reflect patient pathology due to inadequacies of transthoracic echo, an inaccurate or incomplete report, and/or disease progression.[3],[4]

Assessment during OPCABG can be conveniently divided into three phases as assessment before grafting, during grafting, and after grafting. Assessment before grafting should be focused on the primary pathology, but a comprehensive study should be completed in all patients. Baseline assessment of cardiac function before grafting by TEE provides vital information to formulate and manage hemodynamics during OPCABG. It also provides a template with which further assessments can be compared. However, assessment after grafting is solely focused on assessing the results and complications of OPCABG.

Assessment Before Grafting

Systolic function – Global and regional wall motion abnormalities

Assessment of global and regional wall motion abnormalities (RWMAs) is vital in patients undergoing OPCABG. In 2002, the American Heart Association (AHA) Writing Group recommended a 17-segment model for regional wall assessment of the left ventricle (LV) [Figure 1]. This model divides the LV myocardial mass into basal, mid-cavity, and apical thirds.[5] The 17 segments of the LV can be visualized by TEE by 6 views at mid-esophageal (ME) and transgastric (TG) levels. These views provide adequate information to accomplish regional assessment of LV systolic function.

Assessment of global LV systolic function predicts the outcome after OPCABG. It also guides the choice of inotropes, requirement of intra-aortic balloon pump (IABP), and viability of off-pump technique.

Diastolic function

Diastolic dysfunction is the earliest indicator of ischemia. Impaired relaxation of the segment involved is the earliest change detected by TEE. It may progress to pseudo-normalization with worsening ischemia. Advanced stages of diastolic dysfunction are predictive of poor long-term prognosis.[6]

Mitral valve assessment

Assessment of mitral valve for ischemic mitral regurgitation (IMR) is of importance considering its impact on the surgical decision. Tomographic planes to assess mitral valve are ME 4-chamber, ME 2-chamber, ME commissural, ME long-axis (LAX) and TG basal short-axis (SAX) views. Assessment of severity of mitral regurgitation (MR) by TEE can be inaccurate in an anesthetized patient.[7] However, the preoperative hemodynamics can be replicated by optimizing the loading conditions with the use of vasoconstrictors. TEE can be utilized to demonstrate the mechanism and consequences of associated MR which can guide the choice of surgical procedure.

Assessment of the aorta

OPCABG requires manipulation of the aorta for proximal anastomosis by partial clamping. In the presence of severe aortic atheromas, these manipulations can result in increased incidence of stroke. TEE helps to identify, locate, and grade atheromas to make necessary modifications in the surgical procedure. TEE can visualize most of the thoracic aorta except for distal ascending aorta and proximal arch as imaging is affected by overlying trachea and left main bronchus.[8] ME aortic LAX and SAX, upper esophageal aortic arch SAX and LAX, and descending thoracic SAX and LAX views can be utilized to visualize thoracic aorta.

Assessment for coexisting abnormalities by comprehensive examination

Apart from performing the focused assessment, it is necessary to evaluate the heart for coexisting abnormalities. AHA has recommended 28 TEE views for comprehensive assessment.[9] Identification of coexisting abnormalities such as atrial septal defects and valvular lesions may necessitate alterations in the surgical procedure being performed.

Assessment during Grafting

Technically, OPCAB surgery differs drastically from that of conventional CABG as the grafts are placed on a beating heart. Positioning of the heart to facilitate grafting on the posterior vessels is referred to as verticalization while positioning for the left anterior descending artery and diagonals is referred to as displacement. These positions can be attained by combination of manipulations such as pericardial retraction sutures, slings, and application of various mechanical stabilization devices such as the Maquet mechanical stabilizer, the Maquet access device (Maquet, Wayne, NJ, USA), and Octopus stabilizer systems (Medtronic, Minneapolis, MN, USA). These manipulations can cause hemodynamic disturbances by compression of chambers, ischemia, and distortion of mitral annular geometry. TEE can monitor patient tolerance to these manipulations in real time.

Conversion to cardiopulmonary bypass

Significant hemodynamic disturbances can occur during OPCABG which may necessitate emergent conversion to cardiopulmonary bypass (CPB). TEE can predict the impending hemodynamic collapse and facilitate early conversion to CPB. Important predictors are persistent RWMAs, deteriorating global LV function, and significant MR.

Diastolic function

Myocardial ischemia causes various changes in mechanical, hemodynamic, and electrical activities of the heart. RWMAs may worsen or may appear if not existent, upon ischemia during OPCABG. However, the first change that occurs with onset of ischemia is appearance of abnormal relaxation.[6] It actually precedes RWMAs and electrocardiographic, pulmonary capillary wedge pressure changes.[10],[11]

Regional wall motion abnormalities

It is important to assess and detect the new or worsening RWMAs during OPCABG. Intraoperative RWMAs can range from mild hypokinesia to dyskinesias [Figure 2]. Transient hypokinesia in the absence of hemodynamic abnormalities and electrocardiographic changes do not represent significant ischemia. Repositioning the stabilizer or verification of correct placement of intracoronary shunts should be considered when new RWMAs are detected. However, persistent intraoperative akinesia and dyskinesia are associated with myocardial ischemia and postoperative morbidity.[12],[13],[14],[15],[16]

Appearance of new or worsening MR causing hemodynamic disturbances during OPCABG can be the result of either onset of acute IMR or due to alterations in the mitral annular morphology caused by positioning for grafting, but MR in the latter disappears by reverting to anatomical position of the heart.

Acute IMR is differentiated by the presence of significant RWMAs and hemodynamic abnormalities persisting with normal position of the heart. Direction of MR jet with RWMAs in corresponding segments can also be used to identify graft dysfunction.

TEE can diagnose hemodynamic instabilities during OPCABG caused by hypovolemia, LV systolic dysfunction, diastolic dysfunction, chamber compression, and MR due to positioning or graft dysfunction. These instabilities are managed accordingly by Trendelenburg position, intravenous fluids, positive inotropes, inodilators, vasodilators, diuretics, repositioning the heart, consideration for conversion to CPB, and graft revision. Response to these interventions can be guided by real-time assessment by TEE.

Assessment After Grafting

Systolic function – Global and regional wall motion abnormalities

TEE can help in predicting the results of OPCABG. Immediate improvements in previously dysfunctional segments have been demonstrated after successful grafting.[17],[18] Furthermore, pregrafting compensatory hypercontractile segments have been shown to have normal function immediately after grafting. Persistent akinesia and dyskinesia are associated with adverse clinical outcomes while the absence of these RWMAs has been shown to be related to postoperative course without morbidity.[19]

Postgrafting diastolic function and mitral valve assessment for new or worsening MR should be done to rule out graft dysfunction or global LV dysfunction.

Assessment of the aorta

Postprocedure, it is crucial to evaluate the aorta at the sites of partial clamping, proximal anastomoses, and descending thoracic aorta in the presence of IABP to rule out dissection or intramural hematomas.

TEE can be used to identify the need of IABP placement.[20] During IABP placement, it is critical to visualize the passage of the guidewire within the aortic lumen to avoid aortic dissection. Accurate positioning of the IABP tip (2–3 cm below the level of the left subclavian artery) can also be verified [Figure 4]

According to multiple studies, risk of complications of TEE varies between 0.6% and 3.5%.[21],[22],[23],[24] However, incidence of major complication such as esophageal perforation is <0.2%.[25],[26] Minor complications such as sore throat and dysphagia are often short lived, and they can be minimized effectively by careful history taking and physical examination to rule out contraindications before insertion. Laryngoscopic insertion, avoidance of excessive manipulation, and reducing the duration of examination also contribute toward minimizing complications.

Conclusion

TEE is an invaluable diagnostic tool for real-time imaging during OPCABG. It is beneficial not only in the intraoperative period but also in the postoperative care units for better patient outcomes.